1. Field of the Invention
The present invention relates to a braking device for a magnetic tape apparatus, particularly to a braking device for halting the feeding of magnetic tape from a supply reel when the amount of magnetic tape remaining on the supply reel is reduced in a fast forward mode.
2. Description of the Related Art
Well known braking devices for magnetic tape apparatuses are disclosed in JP-A-3-8160 (conventional example 1) and JP-A-5-114199 (conventional example 2).
According to conventional example 1, various operating modes are selected depending on the position of a plate cam. In a fast forward mode, the plate cam renders a braking device inactive in accordance with an energization instruction provided for an electromagnet. When a tape end detector detects a terminal end of a tape, a detection signal is transmitted to a system controller. The system controller then simultaneously outputs an instruction to halt a capstan motor that is rotating a reel table and an instruction to stop the supply of power to the electromagnet. Accordingly, the capstan motor is halted, and the plate cam is displaced by a spring within a short period of time to thereby activate the braking device. This braking method is so-called a quick braking method.
According to conventional example 2, a braking unit is described in which the operating mode of a magnetic recording/reproducing apparatus is switched from a fast forward/fast rewind (FF/REW) mode to a stop (STOP) mode. The magnetic recording/reproducing apparatus according to conventional example 2 includes: a swing type idler gear for selectively switching a first path along which the rotational power produced by a capstan motor is transmitted by gears to a take-up reel table and a second path along which the rotational power produced by the capstan motor is transmitted by gears to a supply reel table; and a clutch for switching between a slide rotation mode during which the rotational speed of the idler gear is synchronized with the rotational speed of a capstan shaft that is limiting the speed at which a magnetic tape travels, and a direct-coupled rotation mode during which the idler gear is directly coupled with the capstan motor. In the fast forward (FF) mode, the clutch is maintained in the direct-coupled rotation mode, and the idler gear is shifted toward the first path to transmit the rotational driving force to the take-up reel table. When an instruction to halt the tape is issued in the fast forward mode, the capstan motor begins to rotate in reverse and the idler gear is shifted toward the second path while the clutch is maintained in the direct-coupled rotation mode.
According to this technique, when the capstan motor begins to rotate in reverse, the direction in which the idler gear rotates is changed and the idler gear is shifted to the second path. Therefore, since the idler gear engages with a gear provided on the supply reel table to prevent the idling rotation of the supply reel table, the quick brake method in conventional example 1 need not be employed. Further, a time period from the halt instruction is issued to rotate the capstan motor in reverse until the idler gear engages with the gear of the supply reel table can be set shorter than a breaking period required for pressing a brake band or a brake shoe against the supply reel table.
According to the quick brake method in conventional example 1, in the fast forward mode, the tape travel can be halted within a short period of time. However, there is a limit imposed on a reduction of a time period required to displace the plate cam when the braking device is changed from the inactive state to the active state. Also, since friction is used to halt the rotation of the supply reel table, the idling rotation of the table cannot be halted immediately when the braking device is activated. Therefore, in the case where the quick break method is employed to immediately halt the tape travel when the tape end detector detects the terminal end of the magnetic tape in the fast forward mode; there is a fear that a leader connected to the end of the tape will be halted immediately before reaching a rotation cylinder of a magnetic head and a boundary between the leader and the magnetic tape rubs and adversely affect the magnetic head.
On the other hand, according to conventional example 2, upon reception of a tape end detection instruction in the fast forward mode, the reverse rotation of the capstan motor is initiated to change the rotational direction of the idler gear, and the idler gear engages with the gear of the supply reel table to prevent the idling rotation of the supply reel table. Therefore, the idling rotation of the supply reel table can momentarily be halted. As a result, the problem in conventional example 1, in which the boundary between the end of the magnetic tape and the leader rubs and adversely affects the magnetic head, can be avoided.
However, in conventional example 2, there is no description about means for preventing magnetic tape discharge phenomenon which reactively occurs in the take-up reel table when the idler gear engages with the gear of the supply reel table and the idling rotation of the supply reel table is immediately halted, i.e., a phenomenon in which the take-up reel table is rotated in reverse in reaction to an extremely large tension force applied to the magnetic tape and the magnetic tape wound around the take-up reel table goes back when the rotation of the supply reel table is halted immediately.
Therefore, using the techniques described in conventional example 2, loosen of the magnetic tape, which can occur due to the magnetic tape discharge phenomenon, cannot be prevented. Further, no description is given of the timing whereat the direct-coupled mode of the idler gear is released. Thus, after the fast forward mode is changed to the stop mode, it is unknown what kind of means should be employed for changing the stop mode to a recording/reproducing mode at normal speed.